Systems are known for detection of intrusion within a protected zone. Many such systems employ Doppler techniques in which a signal is propagated within a zone under surveillance, and signals returned from the zone and from objects therein processed to determine intruder presence by a Doppler signal detectable as an indication of intruder presence. A major requirement of all practical intrusion alarm systems is the reliable discrimination between an actual intruder and noise or other spurious conditions which could give rise to a false alarm indication of intrusion. Sophisticated electronic signal processing circuits have been developed for providing such discrimination, and such processors are a relatively expensive portion of an overall system. For many installations, multiple zones are under surveillance and a common signal processor is employed to receive signals from each of the zones. However, this type of system will provide an alarm when any zone is intruded, but the alarm will not be indicative of the particular zone in which intrusion has occurred. In order to provide identification of the particular zone being intruded, systems of conventional construction employ individual signal processors for each zone, which can materially add to the cost and complexity of an installed system.